1. Field of the Invention
The present invention relates to an optical and electrical mixed flexible printed wiring board including an optical waveguide having flexibility and a method of mounting a light receiving/emitting device thereof.
2. Description of the Related Art
In recent years, miniaturization and higher performance of electronic devices are increasingly promoted and demands for ever-denser circuit boards are growing. Thus, circuit boards are made denser by converting a single-sided circuit board into a double-sided circuit board or multi-layer circuit board having at least three layers.
As part thereof, a hybrid multilayer circuit board as described, for example, in Japanese Patent No. 2631287 (Page 4, FIG. 5) is widely used mainly in small electronic devices such as notebook computers, digital cameras, mobile phones, and game machines. The hybrid multilayer circuit board has a configuration in which multilayer circuit boards or hard circuit boards on which various electronic components are mounted are connected by a flexible cable portion integrating a separate flexible printed wiring board or a flexible flat cable via a connector or the like.
Particularly, the amount of information of these devices is increasing and the signal transmission speed at which such information is transmitted tends to become increasingly faster. The transmission speed of personal computers has shifted to the transmission standard of 6 Gbps in 2010 to 2011 and it is becoming increasingly important to take transmission losses in lines into consideration.
Further, to transmit a pulse signal at high speed, the signal amplitude voltage of a signal source tends to become lower, making correct signal transmission more vulnerable to spike noise originating externally or internally. A board performing high-speed signal transmission needs a characteristic impedance matched transmission line, which makes transmission losses less tolerable.
It is necessary to take noise resistance measures of transmission lines and inside devices against the above spike noise and a shield needs to be formed for the transmission lines. Thus, the transmission line becomes thicker and it may be difficult to ensure flexibility of a hinge connecting the display of a notebook computer or the like and the keyboard. Accordingly, the application of high-speed signal transmission by an optical signal using an optical fiber commercially used in the field of long-distance transmission is examined for small devices to solve problems of losses and noise resistance to transmit an electric signal at high speed.
Further, a transmission line combining an organic polymeric optical waveguide having flexibility with a flexible printed wiring board to be able to apply to the above hinge portion or the like is described in Japanese Patent Application Laid-Open No. 2009-58923 (Page 17, FIG. 11) and Japanese Patent Application Laid-Open No. 2010-286777 (Page 13, FIG. 5).
However, a transmission line combining a flexible printed wiring board made of a resin material of low elastic modulus and heat resistance and a polymeric optical waveguide is subject to tight constraints of the method and conditions for mounting an optical device that emits or receives light. It is also necessary to align each member, to form a 45° mirror to the polymeric optical waveguide, and to protect the optical path and mirror surface and thus, complex processes are needed and it is structurally difficult to manufacture the transmission line with high yields at low cost.
From the above, a structure that enables the manufacture of an optical and electrical mixed flexible printed wiring board including an optical waveguide at low cost with stability is desired.
FIGS. 4A to 4E are structural sectional views schematically showing a conventional an optical and electrical mixed flexible printed wiring board including an optical waveguide described in Japanese Patent Application Laid-Open No. 2010-286777.
That is, a light emitting device 102 including an electrode portion 102a and a light emitting portion 102b and a light receiving device 103 including an electrode portion 103a and a light receiving portion 103b are mounted on one surface of a flexible printed wiring board 101 at predetermined intervals.
The flexible printed wiring board 101 is provided with optical path holes 104, 104 drilled toward the rear surface separately corresponding to the light emitting portion 102b and the light receiving portion 103b and a polymeric optical waveguide 105 having flexibility is pasted to the opposite surface of the double-sided flexible printed wiring board 101 via an adhesive 106.
A mirror portion 107 is formed on an extension line of a core 105a of the optical waveguide 105. A relatively cheap VCSEL (Vertical Cavity Surface Emitting LASER [emitting light in a direction perpendicular to the semiconductor wafer substrate]) is used as the light emitting device 102 and a photo diode having sensitivity at wavelength 850 nm is used as the light receiving device 103 by fitting to the wavelength 850 nm of VCSEL.
The electrode portions 102a, 103a of the light emitting device 102 and the light receiving device 103 are provided on the side of the light emitting portion 102b and the light receiving portion 103b respectively and are mounted on the flexible printed wiring board 101 by the flip chip bonding method by using a solder bump 109.
The light emitting device 102 and the light receiving device 103 are mounted, as shown in FIGS. 4B to 4E, according to the following procedure. The mounting procedure is the same for the light emitting device 102 and the light receiving device 103 and thus, the light emitting device 102 and the light receiving device 103 are denoted as a light emitting/receiving device 123 below.
i) The light emitting/receiving device 123 is recognized by a camera 111 such as a CCD before pickup (see FIG. 4B).
ii) Two opposed side faces of the light emitting/receiving device 123 are picked up by a pickup tool 112 to move from a tray stocking the light emitting/receiving device 123 into a mounting apparatus (here, a flip chip bonder) (see FIG. 4C).
iii) The pickup tool 112 reversely rotates by 180° so that the light emitting/receiving device 123 is passed to a suction nozzle 113 for mounting (see FIG. 4D).
iv) The light emitting/receiving device 123 is sucked by the suction nozzle 113, recognized by a camera 114 (see FIG. 4E), and moved to a mounting stage before being mounted on a board.
It is necessary to protect the lower space of the light emitting device 102 and the light receiving device 103, the inside of the optical path hole 104, and the mirror portion 107 and each is sealed by using a transparent sealing resin 108. If the refractive index of the sealing resin 108b is high, the mirror portion 107 does not function as a mirror and thus, it is also necessary to form a metallic thin film by the sputter process, the evaporation method or the like in the mirror portion 107. Both of the flexible printed wiring board 101 and the optical waveguide 105 have flexibility and so can be bent.
In the conventional structure, the optical axes of the light emitting portion 102b, the optical path hole 104 on the side of the light emitting device 102, the mirror portion 107, the optical waveguide 105, the optical path hole 104 on the side of the light receiving device 103, and the light receiving portion 103b need to be with accuracy up to 30 to 50 μm to be able to transmit an optical signal with stability and it is very difficult to attain such accuracy when a flexible printed wiring board whose dimensions are likely to expand or contract is used.
In addition, optical transmission losses increase in the resin portion sealed in the lower space of the light emitting/receiving device 123 and inside the optical path hole 104 and thus, it is necessary to increase power applied to the light emitting device 102 for signal transmission, which is disadvantageous from the viewpoint of decreasing power consumption.
In contrast, Japanese Patent Application Laid-Open No. 2010-26508 (Page 16, FIG. 1) describes an optical transmission module that does not form a mirror in an optical waveguide, but problems are posed in the following points.
FIG. 5A is a structural sectional view showing the structure of an optical transmission module that does not form a mirror in a conventional optical waveguide described in Japanese Patent Application Laid-Open No. 2010-26508.
A light emitting device 202 and a light receiving device 203 are of end face entry/emission type, which is more expensive than a light emitting device of surface emitting type such as VCSEL. Also in this case, a solder bump 209 is used for mounting and it is difficult to attain accuracy of position in the direction of bump height with respect to an optical waveguide 205. If estimated conservatively, the position fluctuates in units of 10 μm. The method of ensuring the accuracy of position in the XY directions is described, no description is found regarding the Z direction.
As the method of ensuring the accuracy of position in the XY directions, cutting the position of the optical waveguide 205 in the XY directions by laser after the light emitting device 202 and the light receiving device 203 are mounted is described. According to this method, because the shape of the sidewall of the optical waveguide 205 is not good, which increases losses, and the processing accuracy of position by laser is at least a few μm or more, losses are also caused here. In addition, the process is complex.
Further, Japanese Patent Application Laid-Open No. 2008-10837 (Page 18, FIG. 6) describes an optical transmission module that uses a light emitting device of surface emitting type and does not form a mirror in an optical waveguide, but problems are posed in the following points.
FIG. 5B is a structural sectional view schematically showing the structure of an optical transmission module that uses a light emitting device 302 and a light receiving device 303 of conventional surface emitting type described in Japanese Patent Application Laid-Open No. 2008-10837 and does not form a mirror in an optical waveguide 305 and FIG. 5C is an enlarged view of a light emitting device mounting portion.
The light emitting device 302 and the light receiving device 303 use devices of surface emitting type and surface receiving type and the device is connected to a sub-mount board 309 and when the sub-mount board 309 is mounted on a base 310, an optical axis L is adjusted to be directed in the horizontal direction.
There are three pending problems with this approach.
The substantial thickness of a light emitting portion 302b and a light receiving portion 303b is made thicker by using the sub-mount board 309, making it difficult to make the thickness as an optical transmission module thinner.
When connecting the light emitting device 302 and the light receiving device 303 to the sub-mount board 309, it is recommended not to use sealing of resin at this point for convenience in subsequent processes, though at least one of two electrodes is connected by wire bonding 308. If the recommendation is followed, however, the wire 308 may be broken before the sub-mount board 309 is mounted on the base 310 and sealed, which is not preferable.
When connecting the light emitting/receiving devices 302, 303 to the sub-mount board 309, at least one of two electrodes in each device is connected by wire bonding and thus, the distance between the light emitting/receiving portions 302b, 303b and the end face of the optical waveguide 305 increases by the loop of the wire 308 and transmission losses are also caused here, which is disadvantageous from the viewpoint of decreasing power consumption.
Because the accuracy of the optical axis L of the light emitting device 302 and the light receiving device 303 changes depending on mechanical accuracy of the end face of the sub-mount board 309, it is necessary to work on the outer shape of the sub-mount board 309 with high accuracy, increasing the cost of producing the sub-mount board 309.
When a sealing medium 307 is used for fixing, the optical axis L of the light emitting device 302, the light receiving device 303, and the optical waveguide 305 connected to the sub-mount board 309 is aligned and then fixed by the sealing medium 307 at a time and there are two pending problems with this approach.
First, it is disclosed that the alignment may be active or passive, if active alignment should be performed in the configuration, it is necessary to pass a current while the wire 308 is exposed on the sub-mount board 309 and there is still the possibility of breaking of the wire 308 and alignment is considered to be practically difficult.
For passive alignment, the optical axis L needs to be fitted to the predetermined alignment and targets and if an alignment mark is created on the optical device side, recognition from the direction of the optical axis L is needed, which is difficult to implement. For alignment from the surface direction, it is difficult to define the height of the light emitting portion 302b and the light receiving portion 303b of the light emitting device 302 and the light receiving device 303 and the height of the optical waveguide 305. From the above, it is necessary to allow for a certain level of position deviations, which is disadvantageous from the viewpoint of decreasing power consumption.
The method is considered to be valid only if, after the optical axis L being aligned, sealing is done at a time while the optical axis L is held, but how to temporarily fix the sub-mount board 309 and the optical waveguide 305 is not sufficiently described and no description of addition of an adhesive layer is found, which makes the implementation thereof difficult.
Thus, replacing a light emitting device and a light receiving device of an optical transmission module like Japanese Patent Application Laid-Open No. 2010-26508 and Japanese Patent Application Laid-Open No. 2008-10837 in which no mirror is formed by a device having electrodes on the surface opposite to the light emitting/receiving portion can be considered, but no suitable small device is found, making it difficult to realize a thin optical transmission module.
For example, an LED package as described in Japanese Patent Application Laid-Open No. 10-294496 (Page 3, FIG. 1) is available, but even a small-sized package has large dimensions of the length 1.6 mm×width 0.8 mm×thickness 0.25 mm and the thickness of an optical transmission module mounted with the package becomes 0.8 mm so that the optical transmission module cannot be made thin and small.